This invention relates to a method of concentrating solutions of methylene diphenyl diisocyanate. More particularly, this invention relates to a method of concentrating solutions of methylene diphenyl diisocyanate in diphenyl oxide.
The compound 4,4'-methylene diphenyl diisocyanate is an extremely important intermediate for the production of polyurethanes. Commercially, this compound may be made by the thermal decomposition of 4,4'-methylene dimethyl diphenyldicarbamate, preferably as a dilute solution in diphenyl oxide, whereby the reaction mixture contains both the desired diisocyanate product and unreacted dicarbamate. Therefore, in this process, it is necessary to provide a method of separating the diisocyanate from unreacted dicarbamate and solvent or diluent. However, prior to separation of the desired diisocyanate product, the reaction mixture needs to be concentrated with respect to the desired product, 4,4'-methylene diphenyl diisocyanate, because of the large amount of diluent in the reaction mixture.
Prior-art methods for concentrating solutions of methylene diphenyl diisocyanate generally include distillation of the solvent. Such distillation requires rather high temperatures, whereby the desired product is subject to undesirable thermal degradation or formation of higher polymers; or comparitively high vacuum, which adds to the expense of the distillation step while reducing the efficiency thereof. It is therefore important to provide a method of separation which is not subject to these disadvantages.